You are thinking too hard about labeling something as "cheating". What will you do with this situation? Take a vote online and forgive him if 60% of people say "not cheating" but fight with him if it's the other way?

He is lying to you and it hurts you. You should tell him this directly. If he cares about you, this alone should be reason for him to change his behavior.

You both have to agree about what is his and yours approach to porn. Maybe you both can enjoy it, maybe you don't care, maybe you it's good idea to limit it. Maybe you think the amount or kinds of porn he watches are unhealthy. At some point it's possible you two are incompatible if the disagreement is too big.

At the same time, don't expect zero porn for your partner, that's very rare. One thing that I can imagine is that you previously said you want him to never watch porn. He agreed because he was scared, but he isn't actually able to do it. So now you're mad that he watched porn and that he lied about it.